1. Field of the Invention
This invention relates to novel blends of polymers and is more particularly concerned with blends comprising certain copolyamideimides with certain copolyimides.
2. Description of the Prior Art
In U.S. Pat. No. 3,708,458 there is disclosed a series of copolyimides which are derived from benzophenone tetracarboxylic acid dianhydride (BTDA) and either a mixture of 4,4'-methylenebis(phenyl isocyanate) and toluene diisocyanate I2,4- and/or 2,6-isomers) or a mixture of the corresponding diamines. A particularly preferred class of copolyimides arising from that disclosure contains from about 70 to about 90 percent of the recurring units derived from tolune diisocyanate or the corresponding diamine while the remaining 10 to 30 percent of the recurring units are derived from 4,4-methylenebis(phenyl isocyanate) or the corresponding diamine.
Amongst the advantageous properties of the above copolyimides, including their excellent resistance to high temperatures, is their good solubility in dipolar organic solvents, particularly the preferred class of polymers defined above. However, because of the stiffness of the polyimide backbones of the polymers, properties such as the elongation and tear strength of films made therefrom are sometimes of lower values than are generally desired.
U.S. Pat. Nos. 3,717,696, 3,929,691, and 3,843,587 disclose various aromatic polyamideimides which contain amideimide linkages arising typically from the reaction of mixtures such as trimellitic anhydride and terephthalic acid or isophthalic acid with 4,4-methylenebis(phenyl isocyanate); or more complex copolyamideimides arising from mixtures which contain, in addition to the above typical ingredients, a further aromatic dianhydride, and even a second aromatic diisocyanate ingredient (see U.S. Pat. No. 3,843,587).
These aromatic pilyamideimides are characterized by good properties, however, their glass transition temperatures, and thus their ultimate resistance to elevated temperatures, are not always as high as is desirable.
We have now discovered that, by simply blending a copolyimide in accordance with U.S. Pat. No. 3,708,468 with a copolyamideimide in accordance with the prior art, the elongation and tear strength of articles, particularly those of films prepared from such blends, are greatly enhanced over the corresponding values for articles made solely from the copolyimide.
More surprising is the discovery that physical properties such as propagating tear strength and tensile modulus values for films prepared from 50/50 w/w blends in accordance with the present invention have values greater than either one of the blend components taken alone.
Further, the softening point, and thus high temperature resistance, of articles prepared from blends in accordance with the present invention is higher when compared to the softening point of the polyamideimide alone.